Some kind of stealth field? What else can the make this field project? They could be anyone!
Great work, just the way I like my Alphas (I have a soft spot for them, in my mind they are still loyal to the Emperor, just waiting for the Emperor to ascend to true godhood. Then they will be following the correct 'chaos god' :P).
Per the lore noted in the novel "Sons of the Hydra", the Legion's armor DOES have the ability to shift color pattern on account of the scales flipping to a desired color scheme (or at least some can). Specific honors or markings unique to the Chapter require a Librarian/Sorcerer to generate them from nearby minds to complete the illusion.